Attenuators are widely used in electronic circuits to reduce the level of signals. For example, attenuators are used to reduce the level of voltages in measuring instruments to prevent overloading, for scaling, etc. In addition, attenuators are used in amplification systems to control the level of the signal to be ammplified and/or the amount of signal feedback. In audio systems, attenuators are used to control the level of the audio signal emitted by the speaker, earphones or other electro-acoustic transducer.
The most basic form of an attenuator is two components connected in series across the source of the signal to be attenuated. In a resistance attenuator, the two components are, of course, resistors. In a capacitance attenuator, the two components are capacitors. In some attenuators, both resistors and capacitors are connected together to provide an AC attenuator operable over a relatively wide frequency range.
In the past, one of the problems with resistance attenuators, when utilized to attenuate AC signals, is their frequency nonlinearity. More specifically, as the frequency of the AC signal increases, parasitic capacitive coupling to the resistors forming the attenuator has an increasingly greater effect on the attenuation value of the attenuator. At some point, the attenuation value begins to change significantly with changes in frequency. While combined resistance/capacitance attenuators reduce this problem, even such attenuators have a dip in their attenuation value in the region where the attenuator shifts from a predominantly resistance attenuator to a predominantly capacitance attenuator.
Obviously, an attenuator whose attenuation value changes with frequency is undesirable, particularly in the case of precision equipment, such as AC voltage measuring equipment, wherein the maintenance of the same attenuation value over a wide frequency range is vital if accurate results are to be achieved.
Various proposals have been made to overcome the parasitic capacitive coupling disadvantage of simple resistive attenuators so that they can be utilized as AC attenuators. One proposal comprises shielding the higher value resistor with the same potential at both ends. The difficulty with this method is that it creates a distributed R-C network that is theoretically incorrect. As a result, it is difficult to use this proposal to create useful attenuators. Another proposal is to enclose the entire assembly in a conductive box. While this proposal reduces parasitic capacitive coupling, enough parasitic capacitive coupling remains to prevent the attenuation value from remaining the same over a wide frequency range. Moreover, theoretically, the box must be made infinitely large to work satisfactorily. Where size is a significant factor (as it is in microelectronics), this solution is also unacceptable for this reason. A further alternative is to surround the attenuator with a capacitive divider formed of small capacitors. This proposal has the disadvantages that it significantly reduces input impedance and requires a large capacitance to be effective. Reduction of input impedance creates an undesirable loading of the signal source, and large capacitances are undesirable in microelectronic environments. A still further proposal for maintaining the attenuation value the same over a wide frequency range is to guard the attenuator with an external signal. This proposal is undesirable because it requires that the system have the capability of generating highly accurate AC voltages. In summary, none of the prior art approaches to forming an AC resistance attenuator suitable for attenuating AC signals by the same amount over a relatively wide frequency range have been satisfactory. Either the prior art techniques have required costly additional circuitry or they have been unduly bulky. The present invention is directed to providing an AC resistor attenuator that has the ability to attenuate AC signals over a relatively wide frequency range without either of these (or other) disadvantages.
It is an object of this invention to provide a new and improved AC resistor attenuator.
It is another object of this invention to provide an AC resistor attenuator having low parasitic capacitive coupling and, thus, a constant attenuation value over a relatively wide frequency range.
In the past, resistor attenuator networks have been used in amplifier systems to control various signal levels, such as the level of the input signal to be amplified or the level of a feedback signal, for examples. Such resistor attenuators have not been as satisfactory or desirable, particularly when the amplifier system is required to amplify signals over a relatively wide frequency range. Again, the difficulty arises because of the parasitic capacitive coupling that occurs between the resistors forming the attenuator and surrounding items. More specifically, as the frequency of the signal to be amplified increases, the effect of parasitic capacitive coupling increases to change the attenuation value of the resistors forming the attenuator. As the attenuation value changes, the amount of signal amplification changes (or, in the case of an input attenuator, the level of the input signal being amplified changes), which is unacceptable in environments where a constant amount of amplification over a wide frequency range is required.
Therefore, it is another object of this invention to provide an amplifier system suitable for amplifying AC signals by a constant amount over a relatively wide frequency range.
It is a still further object of this invention to provide an AC amplifier system including a resistor attenuator with low parasitic capacitive coupling.